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Contemporary Straight Wire Biomechanics PowerPoint Presentation

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Description : Contemporary Straight Wire Biomechanics

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Contemporary Straight Wire Biomechanics Done by Salah M. Hafedh Supervised by Dr Mohammed Al - labani
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OUTLINE Straight Wire Appliance Design and Values, Straight Wire Appliance and Self-Ligation, Optimal Bracket Placement, Treatment Mechanics, Stage 1: Leveling and Aligning, Stage 2: Working Stage, Stage 3: Finishing Stage, Future Directions, Summary,
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Introduction It is not only the appliance system you have, but how you use it.” The straight wire appliance (SWA) was developed and introduced by Lawrence Andrews in 1970 with the idea of having an orthodontic fixed appliance that would enable the orthodontist to achieve the “six keys” of normal occlusion in the vast majority of cases in an efficient and reliable fashion. Even though the SWA is 40 years old and has become the most common appliance concept over the past three decades, a review of some of the original concepts on which the SWA was designed and the evolution it has gone through are fundamental to better understand the beauty of this appliance and the treatment mechanics, which we then discuss.
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STRAIGHT WIRE APPLIANCE DESIGN AND VALUES There are a few features that need to be present in an appliance to be considered a true SWA; Each bracket has to be tooth specific and have built-in torque, tip, in/out, and (for the molars) proper offset. The torque has to be built in the base of the bracket, not in the face, and the tip in the face of the slot. The base of the bracket must be contoured mesiodistally and occlusogingivally.
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Andrews thought his appliance could be used to treat a large variety of cases, he introduced a series of additional brackets with different degrees of overcorrection to account for undesired tooth movement that would occur specifically when sliding teeth in extraction cases. Therefore, he introduced more mesial tip and rotation to the canine bracket. Andrews then came out with a line of overcorrected brackets, which he first called extraction brackets and then translation brackets. Andrews’ complete bracket system (standard and translation brackets) was less popular than expected, partly because of the large bracket inventory needed to satisfy his treatment mechanics. However, in the mid- 1970s, Ronald H. Roth took the Andrews SWA and combined some of the standard bracket prescription values with some of the overcorrected values found in the translation bracket pre- scription to create the “Roth setup.”
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STRAIGHT WIRE APPLIANCE AND SELF-LIGATION Self-ligating brackets have been classified as “active” or “passive” depending on the behavior of the gate or clip on the archwire. Active SLBs have a clip with a spring effect that exerts pressure on the archwire, pushing it onto the base of the bracket’s slot. This pressure is based on the archwire size and/or bracket and archwire configuration . Passive SLBs have a gate that passively opens and closes without exerting pressure on the archwire. Passive SLBs also have been described as tubes.
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Optimal Bracket Placement Assuming we have the right appliance, the next most important factor when working with an SWA is bracket position. When using an SWA, you “start finishing” your case the day you place the brackets. The FA point is the middle of the clinical crown occlusogingivally and mesiodistally, following the long axis of the crown, for each tooth in the mouth. It is important to emphasize the following concepts: Andrews demonstrated that trained clinicians are able to place brackets consistently at the FA point without any aids but their own eyes.
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The use of any gauge as an aid to position the brackets is not necessary; in fact, to use any predetermined height from the incisal edge to locate the brackets, as some orthodontists advocate, is wrong and negates the use of the FA point, which is one of the fundamentals of the proper management of the SWA. However, you have to take into account shorter crowns caused by excessive gingival tis- sue, worn teeth, or fractured teeth that eventually will be restored, so in some of these situations, the brackets will look more incisally or gingivally than they should be. To fully level and express the torque, tip, and in/out of each bracket, the slot of the bracket has to be filled, which requires 0.021- × 0.025-inch stainless steel wire. It is important to know that a 0.019- × 0.025-inch stainless steel wire has about 10.5 degrees of play on a 0.022-inch bracket slot.10 However, this is not the case when using active SLBs.
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Although the FA point and long axis of clinical crowns are key to bracket position, a few specific considerations facilitate bracket placement on certain teeth, such as the upper and lower canines, upper and lower first molars, and sometimes upper lateral incisors and premolars. Canines: The long axis of the upper and lower canines, which is also the most convex part of the labial surface, is located more mesial than the true mesiodistal center of the tooth; therefore, the FA point looks a little bit more mesial than the dead center of the tooth. If you err and place the bracket on the center of the crown mesiodistally, the canine will rotate mesially. Molars: The landmark that Andrews used as the long axis of the clinical crown for the molar is the buccal groove. The FA point then lies along the buccal groove, midway occlusogingivally (see Fig. 17-2). It is important to realize that the center of the tube mesiodistally should be in agreement with the FA point. As some manufacturers have reduced the mesiodistal length of tubes, orthodontists have started positioning tubes too far mesial, resulting in distal over rotation of the molars.
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Upper lateral incisor: After the third molars, the upper lateral incisors are the teeth with problems involving size and shape. This makes it difficult to determine the long axis of the crown from the buccal. It is wise to use a mirror to look at the lingual surface of the incisor and then extend the long axis of the clinical crown from the lingual to the buccal. Premolars: Usually premolars, specifically second premolars, represent a challenge at the time of bonding because of a lack of direct vision. In these cases, it is advisable to look with a mirror from the occlusal and the buccal to locate the FA point and the long axis of the clinical crown. Both authors believe that indirect bonding can be of great help to better position the brackets, specifically on premolars and molars, decreasing the need for “fine tuning” rebonding toward the end of treatment.
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Treatment Mechanics Simplicity is of paramount importance when teaching; therefore, all the mechanics to be accomplished in our orthodontic treatments with the SWA can be divided into three stages: Stage 1, leveling and alignment; Stage 2, working stage; Stage 3, finishing stage. During each of these stages, there are specific movements of teeth that will occur and specific goals that have to be achieved before continuing to the next stage of treatment. It is important to emphasize that both the treatment outcome and its efficiency will be greatly improved if the orthodontist follows these stages.
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Stage 1: Leveling and Aligning Leveling and aligning is a complex process in which all the crowns are moving at the same time and in different directions. As the teeth level and align, reciprocal forces between them develop, which can be of great help to guide the movements to our advantage. Then, when possible, all teeth should be engaged from the beginning to obtain maximum efficiency of tooth movement. Usually at this stage, round small-diameter thermal-activated wires, such as a 0.014 inch for severe crowding or a 0.018 inch for moderate to minimum crowding, are preferred.
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The following are the movements we should expect and goals we should accomplish when leveling and aligning, before starting stage 2: • Teeth move individually. • It is mainly crown movement. • Molars and Premolar (PM) derotate and upright distally. • Incisors are upright and sometimes even retract. • Start building posterior anchorage. • Before proceeding to stage 2, check bracket position (gross errors) and debond or rebond as indicated. The following are the most common wires and sequence used at stage 1 of treatment: • Mainly round, small-diameter, superelastic wires (ideal thermal activated) • Square or rectangular superelastic wires to correct remaining rotations and level the occlusal plane
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Stage 2: Working Stage This stage of treatment is the one on which we will spend more time. At this stage, the maxillary and mandibular arches are coordinated, proper overbite and overjet are achieved, Class II or Class III are corrected, maxillary and mandibular midlines are aligned, extraction spaces are closed, and maxillary and mandibular occlusal planes are paralleled.
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Arch Coordination. The maxillary and mandibular archwires must be coordinated in order to obtain a stable occlusal intercuspation and proper overjet. In an ideal intercuspation of a Class I, one-tooth to two-teeth occlusal scheme, the palatal cusps of the maxillary molars should intercuspate with the fossae and marginal ridges of mandibular molars, the buccal cusp of the mandibular premolars should intercuspate with the marginal ridges of the maxillary premolars, and the mandibular canines and incisors should intercuspate with marginal ridges of the maxillary canines and incisors. Another important aspect of arch coordination is the effect that it has on the vertical dimension and the sagittal dimension. Arch coordination is a transverse issue. The maxillary teeth should be upright and centered in the alveolar/basal bone and coordinated with the mandibular teeth, which should also be upright and centered in the alveolar/basal bone to obtain a proper intercuspation.
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Overbite and Overjet Correction. An optimal overbite–overjet relationship does not have to be a certain predetermined number of millimeters. More important is the functional relationship they have. This means that the overbite–overjet should be compatible with a mutually protected occlusal scheme and thus allows for a proper anterior guidance in protrusion and lateral excursive movements. Another important factor to consider when evaluating over- bite–overjet problems is the position of the mandible. Often, differences between a maximum intercuspation (MIC) and centric relation (CR) can produce significant differences in the overbite–overjet relationship. Last, but by no means the least important, is the sagittal and vertical relationship of the maxillary and mandibular incisors with the lips.
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Closing Extraction Spaces. Usually after leveling and aligning, the extraction spaces left are smaller than at the beginning of treatment because some of the space has been taken to unravel the initial crowding and to upright the maxillary and mandibular incisors, as described earlier in this chapter. Also, the maxillary and mandibular occlusal planes should be flat or almost flat, and the six anterior teeth should be consolidated into one unit. Then, to efficiently close the remaining spaces, achieving the desired functional and aesthetic goals, we need to determine the anchorage requirement. The visual treatment objective (VTO)is a cephalometric exercise in which we modify the patient’s cephalometric tracing to achieve the desired end-of- treatment result, and then, by superimposing both tracings, we can visualize the movements that need to occur to obtain that result.
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Double Keyhole Loop Activation. The DKH archwire can be activated in two different ways. It can be pulled from the distal of the first or second molar, so as to open the loops 1 mm, and then cinched back to keep the loops open. As the loops close, the teeth come together, closing the space. A different activation method is to open the loops 1 mm and then use a stainless steel ligature to ligate the distal loop to the hook of the first or second molar tube, with sufficient tension to keep the loops open. As the loops tend to close, the ligature will exert force on the molar tube, and the teeth will come together. In either way of activation, changes in wire size and place of activation will determine the type of anchorage obtained. Niti Thermal-Activated Coil Activation. Niti thermal-activated coils come in three different strengths: 100 g (blue dot), 150 g (yellow dot), and 200 g (red dot). It is the authors’ preference to use the 150-g Niti thermal-activated coil. These coils deliver the same force independent of the amount of activation. In our mechanics, we usually crimp a surgical type of hook distal of the canine from which a Niti thermal-activated coil is engaged all the way to the hook of either the first or second molar. If a surgical hook is not available, the Niti thermal-activated coil can be engaged to the hook of the canine’s bracket.
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Minimum Anchorage. On a minimum anchorage situation, molars will be moved mesially to close the remaining extraction spaces. We use a 0.021- × 0.025-inch wire. This wire will express the buccal crown torque of the maxillary incisors and the mesial tip of the canines. In the mandible, this wire will express the mesial tip of the canine. This situation increases the anchorage in the anterior part of the mouth because it would be more difficult to retract or even tip back the anterior teeth while moving the molars forward. It also has been recommended to reduce the size of the wire in the posterior part, specifically the edges, to decrease the anchorage of the posterior teeth. The activation of the DKH or the Niti thermal-activated coils must be done from the first molars. Then, after the first molar has been moved forward as desired, the second molar can be activated and moved forward, too. Often, though, this is not required because the second molars will travel forward as we move the first molars, and then the space remaining between the first and second molars will be very small and easily closed with an elastomeric chain.
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Medium Anchorage. This is the most common anchorage situation encountered in our cases. Medium anchorage means that the remaining spaces should be closed reciprocally. For this situation, we use a 0.019- × 0.025-inch wire. The activation of the DKH or Niti thermal-activated coils is done, most of the time, from the first molar. However, it can also be done from the second molars depending on how the case is progressing. The bone and attachment apparatus are not the same for every patient, and therefore, the response to the closing mechanics could differ among cases. Then a clinical examination of the overbite–overjet, canine and molar relationship, and facial aesthetics should be done at each visit to evaluate any changes in activation that may be required. This should not take any extra time because the activation of a DKH or Niti thermal- activated coil is a rather easy procedure.
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Maximum Anchorage. In a maximum anchorage situation, most of the remaining space left after leveling and aligning is closed because of distal movement of the anterior teeth. We use a 0.019- × 0.025-inch wire. The DKH or Niti thermal-activated coil is activated from the second molars. Although not frequently required, auxiliaries to enhance posterior anchorage such as TPB, TADs, or extraoral force (headgear) can be used as needed. Intermaxillary Elastics. Discretion is a good word to describe the use of intermaxillary elastics. We use them and like them, but it is important to understand how they are used to avoid problems. We do not use intermaxillary elastics in the following situations: • Round wires • Initial leveling and aligning, low-deflection wires • To a terminal tooth, last tooth in the arch • In the anterior part of the mouth to close open bites • In the posterior part of the mouth to correct crossbites • For an extended period of time
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We use intermaxillary elastics in the following situations: • At the working and finishing stages • On square or rectangular stainless steel wires • On the buccal side of the mouth, short Class II or III, or triangular verticals The three types of intermaxillary elastics we commonly use are 3/16-inch 4 oz, 6 oz, and 8 oz elastics as well as 1/8-inch 4 oz, 6 oz, and 8 oz. Short means, in a Class II, for instance, from the maxillary canine to the mandibular second premolar in a nonextraction case and to the first mandibular molar in an extraction case.
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The following are the movements we should expect and goals we should accomplish at the working stage before starting stage 3: Movement of group of teeth in all planes of the space: sagittal, vertical, and transverse Overjet–overbite correction Class II and III correction Close all remaining extraction spaces, aligning maxillary and mandibular midlines Finish leveling the occlusal plane Arch coordination
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The following are the most common wires and sequence used at stage 2 of treatment: In nonextraction cases, a 0.019- × 0.025-inch stainless steel wire. Reverse curve of Spee can be manually added to the wire if needed In extraction cases, either a 0.019- × 0.025-inch or a 0.021- × 0.025-inch stainless steel wire depending on the anchorage requirement.
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Stage 3: Finishing Stage At this stage, to place each tooth on its ideal position and flat- ten the occlusal plane, full bracket expression is desired; thus, a larger wire such as a 0.021- × 0.025-inch or a 0.022- × 0.028- inch stainless steel may be required.
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FUTURE DIRECTIONS Although alternative SWA prescriptions for the maxillary anterior teeth such as “high torque” as well as “low torque” have been in existence for many years, it has not been until recently that SWA systems with variable prescriptions and/or a fully customized prescription have gained some traction.
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SUMMARY The advantages of the SWA are unquestionable. At 40 years after its introduction to our specialty, it remains the most popular orthodontic appliance used in the world. But today, at the beginning of the 21st century, the challenge is to integrate the SWA with recent changes in bracket design such as self-ligation and technologically advanced low-deflection thermal-activated archwires to provide orthodontists with a state-of-the-art appliance system that can deliver, through a practical, efficient, and reliable biomechanical system, excellent results for a wide range of dentofacial problems. The objective of this chapter is to delineate some of the basic principles of the SWA, emphasize the importance of optimal bracket placement, and provide readers with the framework of a simple, but complete, biomechanics tailored for active SLBs.
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